Mars:Mars is called the “red planet.” When you see it in the sky, it has a slightly reddish tinge. This reddish color is due to the breakdown of iron-rich rocks, which creates a rusty dust that covers much of Mars’s surface. Mars’s Atmosphere: The atmosphere of Mars is more than 95 percent carbon dioxide. It is similar in composition to Venus’s atmosphere, but much thinner. You could walk around on Mars, but you would have to wear an airtight suit and carry your own oxygen, like a scuba diver. Mars has few clouds, and they are very thin compared to clouds on Earth. Mars’s transparent atmosphere allows people on Earth to view its surface with a telescope. Temperatures on the surface range from -140°C to 20°C.Images of Mars taken from space do show a variety of features that look as if they were made by ancient streams, lakes, or floods. There are huge canyons and features that look like the remains of ancient coastlines. Scientists think that a large amount of liquid water flowed on Mars’s surface in the distant past. Scientists infer that Mars must have been much warmer and had a thicker atmosphere at that time.At present, liquid water cannot exist for long on Mars’s surface. Mars’s atmosphere is so thin that any liquid water would quickly turn into a gas. So where is Mars’s water now? Some of it is located in the planet’s two polar ice caps, which contain frozen water and carbon dioxide. A small amount also exists as water vapor in Mars’s atmosphere. Some water vapor has probably escaped into space. But scientists think that a large amount of water may still be frozen underground.Seasons on Mars: Because Mars has a tilted axis, it has seasons just as Earth does. During the Martian winter, an ice cap grows larger as a layer of frozen carbon dioxide covers it. Because the northern and southern hemispheres have opposite seasons, one ice cap grows while the other one shrinks.As the seasons change on the dusty surface of Mars, windstorms arise and blow the dust around. Since the dust is blown off some regions, these regions look darker. A hundred years ago, some people thought these regions looked darker because plants were growing there. Astronomers now realize that the darker color is often just the result of windstorms.